Method of producing carbon film resistors



Jan. 19, 1965 w. F. O'SHEA 3,165,819

METHOD OF PRODUCING CARBON FILM RESISTORS Filed Feb. 15, 1961 L3 I2 I Ill I INV EN TOR.

NH-Ll HM E O'S HEA BY United States Patent 3,165,819 METHOD OF PRODUCING CARBGN FILM RESISTORS William F. OShea, Hatboro, Pa, assignor to Continental- Wirt Electronics Corporation, Philadelphia, Pan, a corporation of hennsylvania Filed Feb. 15, 1961, Ser. No. 89,543 1 Claim. (Cl. 29155.71)

This invention relates generally to carbon film resistors, and more particularly relates to an improved method of making the same which results in stabilization of the electrical resistance value by preventing changes in resistance due to mechanical shock and contamination.

Deposited carbon film resistors are in widespread use today throughout the electronic industries, and are generally utilized in those applications requiring precision of value and electrical stability, and also where the inductive effects of wire wound precision resistors may pose a problem. The electrical stability of the deposited carbon resistors depends upon a number of factors. One of such factors has been found to be mechanical stabilization of the carbon film which is very thin and inherently fragile, sometimes being only of molecular thickness. It is, of course, apparent that such thin films are readily susceptible to fracture due to normal handling, and in order to ofiset this fragility the carbon film is usually deposited upon a form which provides an underlying rigid film support, the finished resistor being then covered with a protective coating which shields the carbon film from abrasion and mechanical shocks which would tend to rupture the film at one or more points and change the resistance value of the unit.

While protective coatings applied in the usual manner have effected a great improvement in the stability of deposited carbon fihn resistors, such usually applied coatings have not completely solved the problem particularly with regard to resistors made with an end cap construction by means of which the resistor unit is connected into an electrical circuit. Accordingly, it is a primary object of the present invention to provide a novel method of making deposited carbon film resistors which results in complete electrical stabilization of the finished unit.

Another object of this invention is to provide a novel method of making deposited carbon film resistors which effects electrical stabilization thereof by mechanically stabilizing the entire carbon film and preventing the formation of rupturing film stresses in localized areas.

Still another object of this invention is to provide a novel method of making deposited carbon film resistors which in addition to stabilizing the carbon film on the major portion of the body, also stabilizes the carbon film in the regions under the end cap terminals to thereby prevent breakdown of the carbon film due to mechanical stresses transmitted thereto through the end caps.

Yet another object of the present invention is to provide a novel method of making deposited carbon film resistors which provides such resistors with improved resistance to absorption of contaminants, such as moisture, which cause deviations of resistance value beyond acceptable limits of tolerance.

The foregoing and other objects of this invention will become apparent from a reading of the following specification in conjunction with an examination of the appended drawings, wherein:

FIGURES 1 through 4 illustrate in sequence progressive steps of the method of making the improved resistor according to the invention, FIGURES 1, 2 and 4 illustrating various deposition and coating steps, and FIGURE 3 illustrating spiral cutting of the resistor film to provide a given desired resistance, and the application of the metal end caps.

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In the several figures, like elements are denoted by like reference characters.

FIGURE 1 illustrates a deposited carbon film resistor 10 in a stage of partial completion. As shown, the resistor comprises a cylindrical ceramic form or base 11 upon which has been deposited a carbon film 12, the film completely encasing the form 11. Painted, plated or otherwise deposited at opposite ends or" the resistor are a pair of silver end bands 13, the inside surfaces of which are in intimate contact with the underlying carbon film 12. The carbon film deposition and the application of the silver end bands constitute in order the first two steps of the method of making the completed resistor 10. The next step of the method is illustrated in FIGURE 2, wherein a first protective coating 14 is applied to the unit of FIGURE 1 so that the latter is entirely encased therewithin.

Following application of the first coating 14, and as seen in FIGURE 3, a pair of metal end caps 15 are firmly pressed over the ends of the unit so as to cause the inner surface of each end cap to press through the underlying first protective coating 14 and make good electrical contact with the end band 13. In the process of end cap application some of the protective coating 14 originally disposed thereunder squeezes axially over the surface toward the center of the resistor body, as at 16, and forms a good circumferential peripheral seal between the body of the resistor and the annular end of each end cap 15. While in the showing of FIGURE 3, the end cap 15 is illustrated as being in fiat surface contact with the silver end band 13, in actual practice it is not necessary that such flat surface contact actually occur completely circumferentially about the end of the resistor. Completely satisfactory performance may be obtained by securing the end caps 15 to opposite ends of the resistor body by applying radially inwardly directed forces to the end cap annular wall at symmetrically located regions about the end cap periphery, so that a plurality of good contact regions between the end cap 15 and the silver end band 13 is obtained. Of course, the greater the number of such contact regions, the lower will be the effective contact resistance between the cap and the. end band and the better will be the mechanical stability of the end cap relative to the resistor body.

-As also shown in FIGURE 3, the body of the resistor 10 is spirally or helically grooved from one end to the other as shown at 17, the depth of cut being sufficient to penetrate completely downward through the first protective coating 14 and the carbon film 12 so that no bridging of carbon across any groove 17 remains. The spiral cutting is, of course, conventional and provides a relatively long resistance path between the end caps 15, the particular resistance value desired being obtained by the thickness of the carbon film and the pitch of the spiral grooves 17. The spiralled resistor is then usually checked for resistance value to determine whether or not it falls within specified tolerance limits. If it falls without one set of tolerance limits, as for example a one percent tolerance, it may be placed in the next higher tolerance range such as two percent or five percent. Alternatively, of course, the resistor may have some rework done upon it to bring it within a desired tolerance range. Merely by way of illustration, the spiralled film may be abraded to raise its resistance value if too low, or may have one or more grooves 17 backfilled with a resistance or conductive substance to lower the resistance value if too high. Finally, after all resistance value processing has been carried out, a second protective coating 18 is applied to the entire resistor including the end caps 15 to completely seal the unit and provide mechanical stabilization and protection for the spiralled carbon film which has been exposed by the spiralling procedure.

3. The encapsulation of the resistor by the second protective coating 13 is illustrated in the showing of FIG- URE 4.

The manufacturing of deposited carbon film resistors by employing a ceramic or glass form covered by a carbon film and having end bands applied thereto is not per se new, as neither are theaifixing of end caps 15, spiralling of the carbon film as at 17 or covering a finished resistor with a protective coating such as 18. The novel and very important aspect of the method according to the invention resides in the step of the method illustrated in the showing of FIGURE 2, namely, the application of a first protective coating 14 which completely encapsulates the carbon film 12 and end bands 13 prior to atfixing of the end caps 15. In the usual method of making resistors of this type, the application of the first protective coating 14 is carried out after the end caps have been aflixed to the resistor body in contact with the end band 13. In such a method the squeezed out portion 16 of the coating 14 is never created, nor, of course, would the portion 14. of the coating 14 be present between the end cap 15 and the body of the resistor. The improved reliability of resistors made according to the present invention is brought about principally because of the presence of the portion 14' and 16 of the first protective coating 14.

The portion of the protective coating 14 under the end caps 15 provides improved mechanical stability for the carbon film 12 which it covers by providing a bonding agent which holds the film together, and also by providing much better mechanical stability for the end caps 15 because it effectively fills all air spaces or voids and thus prevents physical shifting of the end caps due to handling. Any such shifting of the end cap 15 brings pressure to bear on the underlying carbon film and tends to produce fracture thereof resulting in changes of resistance in the region where the end bands 13 contact the carbon film. Additionally, the protective coating portions 14' and 16 provide a very effective sealing off of the end regions of the resistor from the atmosphere and hence prevent any possibility of atmospheric penetration under the end cap and into the region or" the resistor covered by the coating portion 14'.

Such atmospheric penetration, of course, can occur with resistors made in the usual manner should any break in the outer protective coating, such as 18, occur. Flaws in the outer protective coating are not particularly uncommon and moreover the tendency toward the development of such flaws in the end cap region is aggravated in resistors made in the usual manner by the relative lack of mechanical stability of the end caps due to the absence of a filling material between the body of the resistor and the inside surfaces of the caps. The atmospheric penetration under the end cap just referred to is highly undesirable because of accompanying moisture which in the usual case would penetrate through the unprotected carbon film and possibly even into the ceramic form. Such moisture penetration is deleterious because by migration through the carbon film it eventually works its way into the spirally grooved portion of the carbon film and results in resistance value changes which in critical applications can render an entire electronic apparatus inaccurate or even inoperable. Thus, the application of the first protective coating such as 14 shown in FIGURE 2, before the afiixing of the end caps 15 to the resistor body results in marked stabilization of resistance value both by its sealing effect and by mechanical stabilization of the end cap structure and the underlying carbon film, and additionally materially reduces the probability of flaw development in the final or outer protective coating.

Having now described my invention in connection with a particularly illustrated embodiment thereof, it will be understood that modifications and variations may occur from time to time to those persons normally skilled in the art without departing from the essential scope or spirit of the invention, and accordingly, it is intended to claim the same broadly as well as specifically as indicated by the appended claim.

What is claimed as new and useful is:

The method of making a film type resistor comprising the steps of, coating a cylindrical ceramic form with a carbon film, applying a silver band over the carbon film at each end of the form so that the bands are separated by an intervening unbroken portion of the carbon film, encapsulating the carbon film and silver bands with a mechanically stabilizing bonding agent first protective coating which holds the film together, mechanically securing a metal end cap upon each end of the form in good electrical contact with the silver bands by applying pressure to the cap sidewalls effective to press the same radially inward about the cylindrical surface of the form so that at least portions of the cap sidewalls press through and displace at least a portion of the first protective coating to thereby contact the silver bands while the remainder of the cap inside walls engage said first protective coating to fill substantially all of the space between the inner surface of the end cap and the outer surface of the resistor end upon which it is secured to thereby prevent subsequent physical shifting of the end cap on the resistor to avoid fracture of the carbon film, removing from the form between the end .caps a groove of the carbon film and the overlying first protective coating, and filling the groove with and encapsulating the composite structure in a second protective coating.

References Cited by the Examiner UNITED STATES PATENTS 1,635,184 7/27 Jones 29-155.69 1,859,112 5/32 Silberstein 29-155.7 2,629,166 2/53 Marsten et al. 29-1555 2,693,026 11/54 Kerridge et al 29155.5 2,803,054 8/57 Kohring.

2,816,996 12/57 Kohring 29155.69 2,933,710 4/60 Novak et al. 338-308 2,934,736 4/60 Davis 338-308 3,107,179 10/63 Kohring 29-155.7l

WHITMORE A. WILTZ, Primary Examiner.

RAY WHINDMAN, Examiner. 

